Most of signal processing techniques, as well signal transmission, collection and storage techniques, utilize digital representation of the measured/received analog signals. Accordingly, the analog signals appearing in nature should first undergo analog to digital conversion (ADC). Today's information society owes much of its success to ADC technology. While electronic ADC technology has vastly been improving, progress eventually slowed down. In the last years, ADC speeds merely doubled, falling short of Moore's law. Electronic ADC technology is the bottleneck today between high-speed analog signal acquisition and the accompanying digital signal processing.
The configuration of the state-of-the-art electronic ADC system is schematically illustrated in FIG. 1A. As shown, an input analog electrical signal is received by an ADC unit, which is operated by a clock with a certain sampling speed (corresponding to at least twice the electrical bandwidth, according to Nyquist sampling criterion) and produces an output digital signal being a quantized digital representation of the sampled values with certain accuracy (resolution) defined by effective number of bits (ENOB). FIG. 1B shows a survey of state-of-the-art electronic ADC performance, where x-axis corresponds to the ENOB and y-axis corresponds to the analog bandwidth. Advance along either of these axes is extremely challenging and is mainly limited by the timing jitter of the electrical sampling circuitry.
With today's state of the art for fast sampling, ENOB of 4.5 bits at 10 GHz and 3.9 bits at 18 GHz [1] or 5-bit (˜4 ENOB) at 12 GS/sec [2] can be obtained. Higher resolution conversion occurs at much slower conversion rates. A recent commercial National Semiconductor product (ADC12D1800) provides sampling rates of 3.6 GS/s, achieved by interleaving a pair of ADCs, each operating at 1.8 GS/s at 9.2 ENOB.
It should be noted that the difficulty of attaining high vertical resolution grows extremely rapidly with frequency. Adding ENOBs is an exponentially difficult task, as each extra ENOB means quadrupling the overall SNR from all noise sources.
In an effort to achieve higher-performance ADCs, there have been numerous applications of photonic technologies to the task of A/D conversion, motivated by the slow improvement trend in bandwidth-resolution performance of electronic ADC, and spurred by the unique advantages of photonics. Various photonic approaches are described in the literature [3] aimed at addressing the limitations of electronic ADC in the sampling and/or quantization techniques (which are the two successive stages of generic ADC operation). Photonic ADC systems have been developed capable of performing sampling, quantization, both sampling and quantization procedures, parallelism gain, etc. Many of such examples of photonic analog-to-digital converter are described for example in Reference 3.
As exemplified in FIG. 2, one typical photonic ADC system includes an optical source producing an optical clock signal in the form of pulses at high repetition rate, a modulator, and an optical detection system including a photodiode. In some cases the optical source is directly modulated by the RF signal, but in most cases a mode-locked laser (either solid state, fiber or semiconductor based) is used for the optical source; and an external modulator, usually a Mach-Zehnder interferometer fabricated from LiNbO3, impresses the RF signal on the optical intensity. In a more general RF photonic link, there is a long fiber or an optical processing stage between the electro-optic modulator and the photodiode. The photocurrent generated at the detection of each of the optical sampling pulses with low timing jitter is then quantized by electrical ADC, deriving the digital signal representation of the RF signal.
In the above example, the photodiode must operate as quickly as the sampling pulses incident on it. Likewise, the ADC must perform its conversion operation at the sampling pulse rate. This limitation can be alleviated by distinguishing between sampling pulses, by wavelength as suggested in Ref. [20]. Here N synchronized optical sampling pulse sources are combined in a manner that forms N equidistant pulses in a frame, which then repeats itself. Since the modulator operation is wavelength independent, the sampling pulses carry the information provided by the electrical RF signal driving the modulator, as in the previous case. However, an optical demultiplexer with a single input port and N output ports accepts the sampling pulse sequence and separates these pulses according to wavelength, such that each pulse in the frame emerges in its own output port. A photodiode disposed at each demultiplexer output port (i.e., N photodiodes in total) converts the incident optical pulse to a photocurrent to be subsequently quantized by an electrical ADC (or plurality of N ADCs, each associated with a photodiode). This photonically-assisted configuration advantageously provides for reduced electrical rate detection and quantization by factor N.
Another example of a photonically-assisted ADC system is described in reference [15], which utilizes N continuous wave (CW) laser sources, each with a unique wavelength, and phase modulation. The output of the superimposed CW multi-wavelength sources is phase modulated by the common phase modulator being driven by a sinusoidal electrical signal and then launched into a dispersive device (such as a single-mode fiber). This fiber creates a pulse train by way of phase modulation being converted to amplitude modulation in the dispersive device for each wavelength component, and the pulses walk off due to the dispersion and form a sequence of N optical pulses each with a different central wavelength, as in the previous example. The optical pulses then go to a second modulator, as an intensity modulator for sampling the RF signal, to be followed by detection schemes using a demultiplexer and N photodiodes and ADC.
Most progress occurred in the field of photonic sampling [4-6], capitalizing on the availability of mode-locked-lasers (MLL) with very low jitter (˜fsec) and fast electro-optic modulators that when combined impart the voltage-under-test (VUT) vUT(t) onto the amplitude of each optical sampling pulse vUT(kT), to be further quantized after detection by means of a conventional electronic ADC (the time jitter of electronic ADCs is in the range of 0.1-0.5 psec). An alternative time-stretch architecture provides spectacular performance in the TeraHz range (e.g. [7]), yet only usable for short intervals that need to be carved out from a continuous signal-stream.